1. Field of the Invention
The present invention relates to an imaging apparatus using more than one image signal read out lines for outputting obtained image data.
2. Description of the Background Art
Conventionally, as a means for converting the image data into electric signals, a solid state imaging element such as a CCD (Charge Coupled Device) has been utilized widely.
A conventional CCD typically has a configuration as shown in FIG. 1, which generally comprises an imaging unit 104 in which cells 102 capable of storing received lights in terms of electric signals are arranged in N by M matrix, and a horizontal transmission unit 105 which is a series of M shift registers for outputting the image data collected by the imaging unit 104 in a horizontal direction to an external device. In addition, the imaging unit 104 is further equipped with a vertical transmission unit 103 for each column which transmits the image data collected by each cell 102 on each column of the N by M matrix in a vertical direction to the horizontal transmission unit 105. In such a CCD, the actual image data are converted into the electric signals at the imaging unit 104, so that a number M.times.N of the cells 102 in the imaging unit 104 defines a number of picture elements in the CCD.
In order to display the image taken by such a CCD in a display device (not shown), it is necessary to transmit the electric signals representing the image data collected by the imaging unit 104 from the horizontal transmission unit 105 through a read out line 101 connected to the display device.
This image data output operation is actually carried out in the CCD as follows. First, the electric signal stored in each cell 102 is transferred to the vertical transmission unit 103, row by row. The vertical transmission unit 103 sequentially shifts down the electric signal transferred from each cell 102 to the horizontal transmission unit 105 by utilizing the charge tranfer effect of the CCD elements. The horizontal transmission unit 105 sequentially shifts the electric signal transmitted from the vertical transmission unit 103 to the read out line 101 from which the electric signal is outputted to the external device. Here, after the stored electric signal is transferred to the vertical transmission unit 103, each cell 102 starts storing the next image data by converting the received lights in terms of the electric signals.
When the image data of the cell 102 at the a-th row and the b-th column in the imaging unit 104 is expressed as Sab, the image data of the cells 102 sequentially outputted through the read out line 101 are in an order of (S1n, S2n, --, Smn), --, (S12, S22, --, Sm2), (S11, S21, --, Sm1). The electric signals representing these image data are then amplified by an amplifier (not shown) connected on the read out line 101 and transmitted to the display device.
Now, in order to display the dynamic image on the display device, there is a need to display several tens of still pictures per second in succession, so that the shifting of the image data in the image data output operation at the CCD must be carried out at a high speed. In particular, the shifting in the horizontal transmission unit 105 is required to be carried out at a very high speed. For example, when the image data obtained by the CCD having 2 million picture elements in total are to be transmitted through a single read out line 101 at a rate of 30 frames per second, the image signals has a frequency of approximately 75 MHz, so that the horizontal transmission unit 105 must be capable of carrying out at least 75 million shifting operations per second.
However, to carry out as many as 75 million shifting operations per second presents practical problems related to the increased heat generation and the circuit implementation, so that it is a rather impractical requirement.
In order to cope with this difficulty, there has been devised a method for reducing the number of shifting operations in the horizontal direction by using a plurality of horizontal transmission units. For example, when the read out line is doubled, the frequency of the image signals in each read out line can be reduced to about one half of that required for a single read out line, so that the horizontal transmission units are required to have the shifting speed of only one half of that required in a case using a signal read out line.
A conventional CCD using double read out lines typically has a configuration as shown in FIG. 2. Namely, this double read out line type CCD comprises the imaging unit 104 equipped with the N by M cells 102 and the vertical transmission units 103 substantially similar to those shown in FIG. 1, and two horizontal transmission units 106 and 107, each of which is a series of M/2 shift registers for outputting the image data collected by the imaging unit 104 in a horizontal direction to an external device, through read out lines 108 and 109, respectively.
Here, in the image data output operation, the image data (S1n, S3n, --, S(m-1)n) of the cells 102 of odd columns are transmitted through the vertical transmission unit 103 to the first horizontal transmission unit 106, while the image data (S2n, S4n, --, Smn) of the cells 102 of even columns are transmitted through the vertical transmission unit 103 to the second horizontal transmission unit 107. The first and second horizontal transmission units 106 and 107 sequentially shift the electric signals transmitted from the vertical transmission unit 103 to the read out lines 108 and 109, respectively, from which the electric signals are outputted to the external device. After these shifting operations in the horizontal direction are finished, the shifting operations in the vertical direction are resumed, and this image data output operation is repeated until all the image data of the cells 102 of the imaging unit 104 are outputted.
Now, when such a CCD using double read out lines is employed, the display of the image data obtained by the CCD can be achieved by using a displaying system having a configuration as shown in FIG. 3.
Namely, for the CCD 110 having double read out lines 108 and 109, where the image data (S1n, S3n, --, S(m-1)n) of the cells 102 of odd columns are outputted through the first read out line 108 while the image data (S2n, S4n, --, Smn) of the cells 102 of even columns are outputted through the second read out line 109 in parallel to the output of the first read out line 108 as described above, the display system is equipped with a first amplifier 111 for amplifying the output of the first read out line 108, a second amplifier 112 for amplifying the output of the second read out line 109, a mixer 113 for mixing the amplified outputs obtained by the first and second amplifiers 111 and 112 in a correct order of (S1n, S2n, --, Smn), --, (S12, S22, --, Sm2), (S11, S21, --, Sm1), an analog low pass filter 114 for filtering the output of the mixer 113, and a display device 115 for displaying the amplified, rearranged, and filtered image data supplied from the analog low pass filter 114.
In this display system, the image data of the odd columns and the image data of the even columns are transmitted to the mixer 113 through two different paths, so that when there is a difference in the characteristics any of the horizontal transmission units, read out lines, and amplifiers constituting these two paths, the discrepancy between the image data of the odd columns and the image data of the even columns can cause the appearance of a false image in a form of vertical stripes on the displayed images.
For example, when the gain of the first amplifier 111 is greater than the gain of the second amplifier 112, the image data of the odd columns are more amplified compared with the image data of the even columns, so that when these image data are mixed by the mixer 113 and then directly displayed on the display device 115, the displayed images will have the false image in a form of vertical stripes in which the portions corresponding to the image data of the odd columns appear brighter than the portions corresponding to the image data of the even columns.
Here, it is known that the appearance of this false image in a form of vertical stripes can be prevented by removing a component at a specific frequency called the Nyquist frequency which is the highest frequency in the image data, so that the display system described above is equipped with the analog low pass filter 114 for removing this Nyquist frequency component from the output of the mixer 113, so as to prevent the appearance of the false image in a form of vertical stripes.
Now, in this display system, it is desirable for the analog low pass filter to remove only the Nyquist frequency component while leaving the remaining components of the image data without any attenuation.
However, the analog low pass filter has the frequency characteristic as shown in FIG. 4A, which indicates that the cut-off frequency fC at which the entered signal is attenuated by 3 dB is separated from the Nyquist frequency fN. Therefore, when the image data are filtered through the analog low pass filter 114, the high frequency components necessary for reconstructing the image at high resolution are also attenuated, so that the resolution of the displayed images is lowered. In order to avoid this lowering of the resolution of the displayed images, it is necessary to bring the cut-off frequency fC closer to the Nyquist frequency fN, but such an adjustment of the analog low pass filter has been difficult as the constants associated with elements of the analog low pass filter must be selected very accurately.
In addition, it has been difficult for the analog low pass filter to remove the Nyquist frequency component completely, because of the characteristics of the elements of the analog low pass filter.
Furthermore, when the cut-off frequency fC of the analog low pass filter is brought closer to the Nyquist frequency fN, the group delay in a vicinity of the cut-off frequency fC becomes very large as shown in FIG. 4B, and as a result, the ringing artifact as shown in FIG. 5 appears on the displayed images.
Now, the imaging apparatus shown in FIG. 3 incorporating the CCD using double read out lines and the display system as described above has been utilized widely in an X-ray diagnostic apparatus for obtaining an image of X-rays penetrated through a patient, which requires a relatively large number of picture elements compared with the other medical equipments using CCD. In such an X-ray diagnostic apparatus, the X-rays are irradiated onto the patient, and the X-rays penetrated through the patient are converted into optical images by means of an image intensifier, and then the obtained optical images are imaged by the imaging apparatus as described above and displayed by the display system as described above.
In this X-ray diagnostic apparatus, the displayed images are utilized in making the diagnosis of small tumors within fine blood vessels or internal organs, so that the lowering of the resolution or the appearance of the ringing artifact in the displayed images can affect the diagnosis, and in fact could possibly lead to a delaying of a discovery of the tumor.